Magelona alba, Taylor & Mortimer & Jimi, 2022

Taylor, Abbie, Mortimer, Kate & Jimi, Naoto, 2022, Unearthing the diversity of Japanese Magelona (Annelida: Magelonidae); three species new to science, and a redescription of Magelona japonica, Zootaxa 5196 (4), pp. 451-491 : 466-469

Magelona alba sp. nov.

[Japanese name: Shiro-obi-morote-gokai]

Figs 4 ; 10–11

Type locality: Hokkaido, Japan

ZooBank LSID: urn:lsid:zoobank.org:act:3F3A6E65-3D71-4AD9-A31F-B68A0F6279E3

Material examined. JAPAN, Off Toyoihama , Hokkaido (Holotype, NMW.Z.2022.001.0012, af; paratype, NSMTPol P-889, af), (43.2266, 141.0164), by snorkelling, 19/10/2014, collected by NJ, sandy sediments, 4–5 m depth .

Diagnosis. A slender species. Prostomium longer than wide, with rudimentary prostomial horns. Chaetigers 1–8 with slender, smooth-edged, triangular lamellae, with superior dorsal lobes. Lamellae of chaetiger 9 triangular, without superior dorsal lobes, but with prechaetal lamellae below neurochaetal bundle. Thoracic chaetigers with capillary chaetae only. Abdominal lateral lamellae spatulate. Hooded hooks tridentate, in two groups, vis-à-vis.

Dimensions. A slender species, thorax marginally thinner than abdomen (when viewed dorsally, Figs 10A ; 11B, C ); when viewed laterally the dorsoventrally flattened thorax is thinner than the more rounded abdomen. Difference between thorax and abdomen not marked ( Fig. 10A ). Holotype, anterior fragment: prostomium 1.0 mm long, 0.85 mm wide; thorax 7.0 mm long (including prostomium), 0.5 mm wide (throughout entire thorax); abdomen 0.6 mm wide; total length 14.5 mm for 25 chaetigers (last chaetiger dissected and slide mounted). Paratype, anterior fragment: prostomium 0.75 mm long, 0.60 mm wide; thorax 5.75 mm long (including prostomium), 0.35 mm wide (throughout entire thorax); abdomen 0.4 mm wide; total length 20.0 mm for 41 chaetigers.

Description. Prostomium longer than wide (L:W ratio 1.2–1.25), with rudimentary horns (not separated from anterior margin, but more distinct than a squared anterior margin), anterior margin smooth, rounded triangular ( Figs 10B ; 11A ). Prostomium with two pairs of prominent longitudinal dorsal muscular ridges, relatively thin. Inner pair abutting for two-thirds but diverging at distal tips, outer pair lightly transversely ridged, shorter, and thinner, abutting inners for entire length. Large, rounded, oblong prostomial markings either side of ridges. Burrowing organ ( Fig. 11D, E ) partially everted on both specimens, oval, ridged longitudinally, appearing smoother on upper surface. Shape of full eversion unknown.

Left hand palp retained on both specimens ( Fig. 11B–D ), long (palps reaching approximately 7.0– 8.5 mm long, around chaetigers 16 or 17), slender. Papillae numerous and digitiform ( Fig. 11B, D ); proximally with three to four rows either side of indistinct longitudinal line, medially three rows, and distally with one row either side. Nonpapillated region long, reaching approximately chaetiger 3 on both specimens, when folded backwards ( Fig. 11D ).

Achaetous region behind prostomium approximately twice the size of chaetiger 1 ( Figs 10A ; 11B ). Thoracic chaetigers very long ( Figs 10A ; 11B, C ), longest at chaetigers 5 and 6 (twice as long as wide). Chaetigers 1–8 similar; parapodia biramous with low triangular notopodial prechaetal lamellae confluent with slender triangular lamellae, postchaetal to subchaetal in position ( Figs 10C–J ; 11F ), distinctly subchaetal by chaetiger 7. Postchaetal lamellae increasing slightly in size along thorax to chaetiger 6 or 7. Superior dorsal lobes present on chaetigers 1–8, long, tapering. Neuropodia with single slender triangular ventral lamella, directly below each chaetal bundle, roughly the same size as each other (marginally smaller in posterior thorax), but smaller than notopodia. Ventral lamellae confluent with low pre- and postchaetal ridges, which encircle chaetae cuff-like. Lamellae from chaetiger 6 appearing in a slightly prechaetal position, distinctly prechaetal by chaetiger 8.

Notopodial postchaetal lamellae of chaetiger 9 ( Fig. 10K ) low triangular, smaller than on preceding chaetiger, confluent with, and adjoining low prechaetal lamellae toward distal tips, appearing almost lateral in position. Neuropodia with triangular to digitiform postchaetal lamellae, pointing laterally, and appearing above chaetal bundle. Additional smaller prechaetal lamellae observed on both neuropodial rami of holotype but only on left neuropodial ramus on paratype. Neuropodial postchaetal lamellae of paratype slightly less developed than that of holotype due to its smaller width. Thoracic chaetae simple, smooth-edged, unilimbate capillaries ( Fig. 10M ), marginally longer in the neuropodia until chaetiger 6 where they become roughly equal in length. Those of chaetiger 9 splayed ( Fig. 10K ), distal tips slightly pennoned and bent ( Fig. 10N ). Distinct reniform ventral swellings, level with parapodia from chaetiger 6–8 ( Fig. 11C ), paired.

Abdominal chaetigers with foliaceous lateral lamellae, of about equal size in both rami, basally constricted ( Fig. 10L ). Triangular processes (DML, VML) at inner margins of chaetal rows, fairly long, digitiform. No obvious postchaetal expansion behind chaetal rows. Abdominal chaetae tridentate hooded hooks, superior two fangs parallel above main fang ( Figs 4A, B ; 10O ). Hooks in two groups, main fangs vis-à-vis ( Fig. 10L ). Approximately 6–8 hooks per ramus. Hook adjacent to lamellae marginally smaller in comparison to other hooks. No abdominal support chaetae (‘aciculae’) observed. No pouches observed.

Pygidium unknown.

Colour. No living material observed. Holotype and paratype white in colour after preservation in alcohol. Staining with Methyl Green solid from achaetous region until chaetiger 6, and along the abdomen, whilst chaetigers 6–9 remain relatively unstained, appearing as a solid white band ( Figs 10A ; 11B, C ). Staining pattern stronger on holotype than paratype. Staining comprising of small speckles on dorsal and ventral side, particularly in anterior thorax. Palps carry longitudinal lines of speckled stained, towards the bases of the papillae ( Fig. 11D ). Dorsal and ventral surface of prostomium speckled ( Fig. 11D ). After some time, staining is retained as speckles on palps, and from prostomium to chaetiger 6 (particularly chaetigers 4 and 6); additional interparapodial speckling observed in abdomen. Edges of ventral swellings appearing as an unstained V shape particularly between chaetigers 5 and 6.

Habitat. Type specimens found at 4–5 m depths at Toyoihama , Hokkaido, Japan ( Fig. 1 ) .

Distribution. Magelona alba sp. nov. is currently only known from Japanese waters.

Etymology. The specific name is derived from the Latin ‘ albus ’ meaning white, referring to the distinct white band present in the posterior thorax of stained individuals of this species.

Remarks. Morphologically, M. alba sp. nov. approaches a suite of ten magelonid species: Magelona anuheone Magalhães, Bailey-Brock & Watling, 2018 ; Magelona capensis Day, 1961 ; Magelona filiformis Wilson, 1959 ; Magelona hartmanae Jones, 1978 ; Magelona hobsonae Jones, 1978 ; Magelona kamala Nateewathana & Hylleberg, 1991 ; Magelona noppi Nateewathana & Hylleberg, 1991 ; M. paulolanai ; Magelona pitelkai Hartman, 1944 ; and Magelona sp. K of Uebelacker & Jones (1984).

In terms of prostomial shape, the new species shares morphological similarities with M. hartmanae and M. hobsonae both from the Temperate Northern Pacific, and Magelona sp. K from the Temperate Northern Atlantic. However, M. alba sp. nov. differs from all three species which possess low and broad postchaetal neuropodial lamellae on chaetiger 9, in comparison to the long slender triangular postchaetal neuropodial lamellae of the new species. Magelona alba sp. nov. shares similarities with M. capensis , M. filiformis , M. kamala , M. noppi and M. pitelkai in terms of thoracic lamellar shape, however, these species differ in possessing shorter prostomia with more rudimentary prostomial horns. Magelona pitelkai , also originally described from the Temperate Northern Pacific, further differs from the new species in possessing a triangular postchaetal expansion in the neuropodia of chaetigers 7 and 8, not present in the new species, and broader postchaetal neuropodia of chaetiger 9. Magelona capensis , from Temperate South Africa, differs in possessing more foliaceous thoracic notopodial lamellae, not slender as in the new species. Magelona kamala , from the Western Indo-Pacific, differs in possessing neuropodia of chaetiger 9 which are low and broad. Magelona noppi also from the Western Indo-Pacific shares many similarities to the new species in terms of lamellar shape, however, the neuropodia of chaetiger 8 are similar to chaetiger 9 in possessing both postchaetal and prechaetal lamellae, unlike the new species in which chaetiger 8 resembles those that precede it. Magelona anuheone and M. paulolanai , both from the Eastern Indo-Pacific, differ from the new species in terms of prostomial shape, the former species although possessing a long prostomium has more distinct prostomial horns, and the latter species has rudimentary prostomial horns and a prostomium of similar length to width. Magelona anuheone further differs from the new species in possessing only postchaetal lamellae of chaetiger 9, unlike the new species which has pre- and postchaetal lamellae. In contrast to the new species, Magelona paulolanai , also possesses broad triangular postchaetal neuropodial lamellae on chaetiger 9.

Figures

FIGURE 1. Map highlighting the sampling locations around Japan where all observed material was collected.

FIGURE 4. Tridentate abdominal hooded hooks of Magelona alba sp. nov. (A–B), Magelona cf. cornuta (C–E) and Magelona armatis sp. nov. (F–G). A–C, F & G lateral views; D, frontal view; E, posterolateral view.

FIGURE 10. Magelona alba sp. nov. (NMW.Z.2022.001.0012). A, anterior, dorsal view showing Methyl Green staining pattern; B, prostomium, dorsal view; C–L, left-hand parapodia of chaetigers 1–9, 11 respectively, anterior views; M, unilimbate chaeta from chaetiger 6; N, unilimbate chaeta from chaetiger 9, with marginally pennoned tip; O, tridentate abdominal hooded hook from chaetiger 11, oblique lateral view. Pr = prostomium, PH = prostomial horns, Pp = palp, Noto = notopodia, Neuro = neuropodia, SDL = superior dorsal lobe, TP = triangular processes, numbers indicate chaetiger.

FIGURE 11. Magelona alba sp. nov. (NMW.Z.2022.001.0012, A–F), Magelona cepiceps Mortimer & Mackie, 2006 (NMW. Z.2000.020.0208, Holotype, G & H). A, prostomium, dorsal view; B, anterior, dorsal view; C, entire specimen, ventral view; D, prostomium and first five chaetigers, ventral view; E, thorax, ventral view; F, chaetigers 4 and 5, ventral view; G–H, entire specimen, dorsal and ventral views respectively. B–D, F–H, stained with Methyl Green.Ab = abdomen, BO = burrowing organ, PH = prostomial horns, Pp = palp, Pr = prostomium, Th = thorax, Noto = notopodia, Neuro = neuropodia, SDL = superior dorsal lobe, VS = ventral swellings, numbers indicate chaetiger.